Method and means for sintering materials, particularly dolomite and magnesite, in a shaft furnace

ABSTRACT

A material, such as dolomite or magnesite, is sintered in an oil- or gas- fired shaft furnace by passing the material down the furnace through a calcining zone, a sintering zone and a cooling zone. The material is subjected to the action of gases flowing in counter-current to the material in the calcining zone, is sintered in the sintering zone by gases flowing in co-current, and is cooled in the cooling zone by gases flowing in countercurrent. The exhaust gases are discharged from the lower end of the sintering zone while cooling air discharged from the upper end of the cooling zone is extracted from the furnace and is reintroduced therein together with fuel at the lower end of the calcining zone to provide the gases required to calcine the material. A fuel gas-air mixture containing substantially no exhaust gas is introduced into the upper end of the sintering zone and, depending on whether the fuel gas-air mixture contains an excess of oxygen or fuel, additional fuel or air is introduced into the material in the sintering zone so as to increase the combustion therein and heat the calcined material to the sintering temperature.

United States Patent 1 3,695,595 Beckenbach [451 Oct. 3, 1972 [54] METHOD AND MEANS FOR [57] ABSTRACT SINTERING MATERIALS,

PARTICULARLY DOLOMITE AND MAGNESITE, IN A SHAFT FURNACE [72] Inventor: Karl Beckenbach, Meererbusch Hildegundisallee 33, Buderich/Dusseldorf, Germany [22] Filed: Nov. 30, 1970 [21] Appl. No.: 93,578

[30] Foreign Application Priority Data Dec. 10, 1969 Germany ..P 19 61 789.1

[52] US. Cl ..263/29 [51] Int. Cl ..F27b 1/00 [58] Field of Search ..263/29, 30, 53 R [56] References Cited UNITED STATES PATENTS 3,285,590 11/1966 Parsons ..263/29 3,544,096 12/1970 Buchner ..263/29 Primary Examiner-John J. Camby Attorney-Singer, Stern & Carlberg A material, such as dolomite or magnesite, is sintered in an oilor gasfired shaft furnace by passing the material down the furnace through a calcining zone, a sintering zone and a cooling zone. The material is subjected to the action of gases flowing in counter-current to the material in the calcining zone, is sintered in the sintering zone by gases flowing in co-current, and is cooled in the cooling zone by gases flowing in counter-current. The exhaust gases are discharged from the lower end of the sintering zone while cooling air discharged from the upper end of the cooling zone is extracted from the furnace and is reintroduced therein together with fuel at the lower end of the calcining zone to provide the gases required to calcine the material. A fuel gas-air mixture containing substantially no exhaust gas is introduced into the upper end of the sintering zone and, depending on whether the fuel gas-air mixture contains an excess of oxygen or fuel, additional fuel or air is introduced into the material in the sintering zone so as to increase the combustion therein and heat the calcined material to the sinte'ring temperature.

- PQaiaaPsswiesfisa METHOD AND MEANS FOR SINTERING MATERIALS, PARTICULARLY DOLOMITE AND MAGNESITE, INA SHAFT FURNACE This invention relates to a method for calcining and sintering materialssuch as dolomite and magnesite in a shaft furnace and to shaft furnaces useful for such a method.

Shaft furnaces in which the material being calcined and the calcining gases travel downwards in the lower calcining zone of the furnace (i.e. in co-current) are already used to an increasing extent for calcining lime. in this known method, the downwardly flowing calcining gases substantially comprise an exhaust gas-cooling air mixture which has been drawn off from the lower calcining zone and in which fuel has been burnt with a high excess of air before the mixtureuis reintroduced into the shaft.

This'method of operation is useful for calcining lime because the temperatures in the co-current zone diminish with an increasing limestone neutralization rate. This means that the temperature at the lower end of the co-current zone (for the final carbonizing phase) is lower than in the upper end of this zone. If this were not the case, there would be some danger of the lime produced in this manner being calcined hard rather than soft.

The application of the co-current method appeared to be unsuitable for sintering (a process which is different from the soft calcining of lime) so that such a process has not hitherto been employed for sintering.

Sintering is performed at the maximum possible temperatures and the sintering will be the more complete the more the temperature approaches the softening point of the material being sintered. For example, in sintering dolomite, temperatures of up to l,800 to 2,000C will be employed depending on the purity of the sintered material; substantially lower temperatures are normally required for other material to be sintered, for example ores.

In the counter-current method of sintering, where the fuel gases flow upwardly against the direction of the material passing through the furnace, these gases enter the shaft at their highest temperature. Under these circumstances, it is not possible in practice for the incoming fuel gases tobe immediately distributed with such uniformity that the flow across the entire cross section is uniform immediately after the entry of the fuel gas so that identical temperatures can prevail everywhere; this feature is necessary for the uniform sintering of the material. It is furthermore difficult to heat the fuel gases prior to their. entry into the furnace to the required high temperatures since in the combustion chambers (in which the gas is heated) it is possible for the stability of the furnace lining to be endangered due, for example, to reactive dust deposits onthe combustion chamber walls.

If the fuel is introduced into the furnace in countercurrent to the material being sintered so that afterburning takes place within the materiaLwthere is no assurance that the fuel becomes available immediately at a uniform rate over the entire furnace cross section and that such fuel burns with the same air ratio, particularly in the lower maximum temperature range near the gas entry positions. Accordingly, zones at very high temperatures are produced, such temperatures being higher than the intended sintering temperature and this causes conglomeration or melting a feature which in turn results in irregularities in the air-gas flow and in the quality of the sintered material. This can even lead to fritting of the entire furnace contents and adhesion thereof to the furnace lining.

l have now found that the co-current method of operating a furnace offers substantial advantages for sintering materials provided it is modified in the manner described hereinafter. According to the present invention, therefore, I provide a method for the calcining and sintering of a material such as dolomite or magnesite in an oilor gas-fired shaft furnace, which comprises passing the material down the furnace through, in sequence, a calcining zone, a sintering zone and a cooling zone, the material being calcined in the calcining zone by gases flowing in counter-current to the material, being sintered in the sintering zone by gases flowing in cocurrent, and being cooled in the cooling zone by gases flowing in counter-current again, exhaust gases discharged from the lower end of the sintering zone and cooling air discharged from the upper end of the cooling zone being extracted from the furnace and being reintroduced therein, together with sufficient fuel, at the lower end of the calcining zone to provide the gases to calcine the material, introducing a fuel gas-air mixture containing substantially no exhaust gas into the upper end of the sintering zone and, depending on whether the fuel gas-air mixture contains excess unburnt oxygen or fuel, introducing additional fuel or air into the material in the sintering zone so as to increase combustion therein and heat the calcined material to the sintering temperature.

I further provide a shaft furnace for performing the method of the invention, in which the furnace wall is provided with a plurality of combustion chambers, means for extracting gas from the furnace being disposed below the combustion chambers and communicating with at least some thereof so that extracted gas may be recycled tothe furnace through combustion chambers, and means being provided between upper combustion chambers and the gas extraction means for introducing additional air or fuel to the furnace.

l have recognized that the danger in the counter-current method of the sintered material conglomerating due to irregularities of combustion canbe avoided by applying the co-current method provided that exhaust gas-cooling air mixture is introduced into the calcining zone of the furnace. Sufficient fuel must also be provided to produce a sufiiciently high temperature for calcining of the material, however this temperature must be less than the long-term stability temperature of the lining of the gas entry positions. A fuel gas-air mixture containing little or no exhaust gas and at a temperature below the long term stability temperature of the gas entry position lining is also introduced at the upper end of the sintering zone and depending on whether the fuel gas-air mixture supplied to the system contains excess unburnt oxygen or unburnt fuel, additional fuel or additional air is introduced below the upper calcining zone into the column of material in order to heat the downward flowing fuel gas-air mixture to the sintering temperature.

The method according to the invention ensures that the pre-treated material transferred from the calcining zone into the sintering zone, which operates under cocurrent condition, is raised to the sintering temperature by fuel gases introduced at the upper end of the sintering zone, the temperature of said fuel gases being below the temperature which would endanger the stability of the combustion chamber lining and, for example when sintering magnesite and dolomite (where such temperature is below the required sintering temperature) by the after-burning of additional fuel. The introduction of the fuel gases required for sintering into the sintering zone takes place at the low temperature end of this zone by contrast to the counter-current method in which the fuel gases must be introduced at the high temperature end of this zone. This means that combustion of the additional fuel permits equalization of the temperature across the furnace cross section within the column of material as it moves downwards through the sintering zone and before the highest sintering temperature is reached. It is therefore not possible for the sintered material to be subject to localized overheating which could endanger the shaft lining by being baked onto the furnace wall.

The additionally supplied fuel or the additionally supplied air may be introduced into the column of material either above or below the entry positions for the fuel gas-air mixture disposed at the upper end of the sintering zone.

In a preferred embodiment of the invention, the column of material which moves downwards in the shaft is deflected in the calcining zone from its main direction of movement in order to form a cavity within the material charge at a location below the entry positions for the gas-cooling air mixture. The fuel gas-air mixture employed for sintering and/or the additional fuel or the additional air is then introduced into this cavity.

Advantageously, the exhaust gas-cooling air mixture extracted from the furnace is cooled by heat exchange with air prior to being re-introduced to the calcining zone, the heated air being used in combustion in the calcining and/or the sintering zone.

In a preferred embodiment of the shaft furnace according to the invention, the lower combustion chambers extend into a cavity in the column of material, the cavity being formed by a bridge extending through the shaft or by a flared zone of the furnace cross section. The supply means for the additional fuel and the additional air may also extend into these cavities.

In order that the invention may be more fully understood, it will be described, by way of illustration only, with reference to the accompanying drawings, in which:

FIG. I is a diagrammatic view of a shaft furnace in accordance with the invention, the upper and lower ends of which are not shown in detail;

FIG. 2 is a section along the line 2-2 of FIG. 1;

FIG. 3 is a section along the line 3-3 of FIG. 1;

FIG. 4 is a diagrammatic view of part of a shaft furnace similar to that shown in FIG. 1; and

FIGS. 5 and 6 are diagrammatic views of further means for the introduction of additional fuel or additional air into the column of material passing through the furnace.

The preferably round shaft 10 of the furnace shown in FIG. 1 is only shown partially since the apparatus for feeding the =raw material to the furnace at its upper end and the apparatus for withdrawing the sintered product at its lower end have been omitted in the interests of clarity. The upper end of the shaft 10 illustrated in FIG. 1 is the zone in which the material is calcined (zone A in FIG. I the lower end of which zone is provided with upper combustion chambers 12 extending into the shaft. Four upper combustion chambers 12, disposed in one plane, are uniformly distributed around the shaft circumference. A roasting zone (zone B in FIG. I), i.e. a zone through which no gases flow, is provided below the combustion chambers 12 and a bridge system is disposed in this roasting zone. The bridge system comprises a single bridge 14 extending diametricallyv through the shaft and being capable of deflecting the roasting material laterally. The bridge is laterally expanded into an arch so that the surface of the column of material in the cavity 16 formed below the bridge corresponds to the curved line 18 shown in FIG. 1. The bridge, which may be optionally cooled, is provided on its underside with an upwardly orientated recess 20 (see FIG. 2), the two ends of which communicate with two oppositely disposed lower combustion chambers 22 which are in alignment with the bridge and through which either a fuel gas-air mixture with a high excess of air, or a fuel gas-air mixture with an excess of unburnt fuel, is introduced. The furnace zone C below combustion chambers 22 is the zone in which the material is sintered.

Supply ducts 24 for additional fuel or additional air are disposed below the combustion chambers 22, the

outlets of these supply ducts extending into the cavity 16. If the fuel gas-air mixture discharged from the combustion chambers 22 contains excess oxygen then additional fuel is supplied through the supply ducts 24 (which may be constructed as burners), the additional fuel mixing with the gases discharged from the combustion chambers 22 into the cavity 16 so as to burn with the excess oxygen. If insufficient air is used in the lower calcining zone 0.3415), that is to say when operating with a mixture containing an excess of fuel, additional air is supplied through the ducts 24. In both cases, the temperature of the gases produced in the lower combustion chambers 22 must not exceed the maximum temperature determined by the stability of the combustion chamber linings. The gas temperatures of approximately 1,800-2,000C required for sintering, for example, magnesite or dolomite will be reached only in the event of additional fuel being supplied through the ducts 24. This fuel distributes itself within the cavity 16 and burns partially therein but burns to a greater extent within the column of material which is disposed in the lower zone of the furnace (the sintering zone), with the oxygen obtained from the excess air discharged from the combustion chambers 22. The conditions are substantially the same if additional air is supplied through the ducts 24 in order to burn excess fuel discharged from the combustion chambers 22.

Although a liquid or gaseous fuel is normally used, it is also possible to use fuel in a solid form, for example in the form of coal dust or crushed coke. This may be introduced at the upper end of the sintering zone into the column of material provided that the ducts are suitably constructed.

The gases which travel downwardly in co-current with the material through the sintering zone below the bridge 14 are drawn off through two extraction apertures 26, which are disposed at the upper end of the cooling zone (zone D in FIG. 1), together with the cooling air rising up to the sintering zone from the cooling zone. These apertures 26, which are shown in FIG. 1 as being disposed in the plane of the bridge for simplicity, are offset by 90 relative to this plane as can be seen from FIG. 3. The offset arrangement of the apertures 26 assists in the uniform distribution of the fuel gases in the co-current sintering zone.

Since the fuel gases drawn off from the sintering zone are at a high temperature (corresponding to the tem perature at which the material is to be sintered) and since the amount of cooling air is suitably proportioned so as to be merely sufficient to cool the sintered material in the cooling zone to a temperature which is tolerable for the conveying means to discharge the sintered material from the lower end of the furnace, it is desirable to cool the gas mixture extracted through apertures 26 so that it can be conveyed without difficulty by the extraction elements 27. These extraction elements may take the form of injectors or fans. The extracted gas mixture is therefore preferably first ducted through heat exchangers 28 in which it may be cooled by means of the air supplied to the burners 30 and 32 respectively by means of a blower 34. The air being'supplied to the burners is thus heated in the heat exchanger. It is, however, possible for other types of fluids (e.g. liquids) to be used as coolants in the heat exchangers, such substances being suitable, after heating in the heat exchanger, for other purposes. The cooled gas mixture, or at least a substantial part thereof, is then supplied by the extraction elements 27 to the upper combustion chambers 12 through ducts 36. It is also possible for a small part of the mixture to be supplied through an ad justable duct, not shown, to the lower combustion chambers 22.

The diagrammatic drawing in FIG. '1 shows at the top right-hand side an extraction fan 40 communicating with a chimney. This fan and the extraction, elements may be so regulated that a roasting zone, through which practically no gases flow, is produced in the zone disposed between the upper and lower combustion chambers. It is of course possible however that asmaller part of the fuel gases produced in the upper combustion chambers flows downwards through the roasting zone. For the sake of simplicity, the regulating elements provided in the ducts are not shown in FIG. 1.

FIG. 4 shows an embodiment of the invention in which lance-shaped ducts 24a for the supply of additional fuel or additional air pass through the upper wall of the combustion chambers 22a. The path of the fuel or air supplied in this manner into the cavity 16a provided below a bridge 14a is shown by the dot-dash lines. The bridge is provided with a cooling duct 17. The supply lines for delivering coolant to this duct are not shown.

FIGS. 5 and 6 show in purely diagrammatic form further means for disposing supply lines to the furnace for additional fuel or additional air. FIG. 5 indicates the entry of the supply lines 24b 'into the cavity disposed below a bridge 14b, extending through the column of material between the upper or lower combustion chambers 12b or 22b respectively. In this case, the lower combustion chambers 22b extend into an annular cavity 16b which is formed by a flared zone of the shaft cross section.

FIG. 6 shows the injection under high pressure of additional liquid fuel or the blowing of additional gaseous fuel at 24c into the shaft disposed between the upper and lower combustion chambers or 22c respectively. The injection of additional liquid or gaseous fuel or additional air may, of course, also be performed below the lower combustion chambers 22, a feature which is also indicated in FIG. 6.

Injection of liquid fuel into a zone of the shaft disposed between the upper and lower combustion chambers only takes place if the zone is either a roasting zone or if the gases in the zone flow downwardly; gaseous fuel or air are blown into this shaft zone only in the last-mentioned case.

The supply of additional fuel or additional air is performed preferably at several positions distributed around the shaft circumference and, where appropriate, also from positions disposed at different levels.

If a material such asdolomite is to be sintered. by the method of the invention, the dolomite is hard calcined (death-burned) in the upper part of the furnace which operates by the counterflow method or is so calcined that all or the greater part of the carbon dioxide is driven out. The sintering process in the co-current sintering zone is initiated by gases supplied via the lower combustion chambers at the upper end of the sintering zone at a temperature which is still below the temperature which would endanger the lining of the gas entry positions or of the combustion chambers. Combustion of the additional fuel supplied to the upper end of the sintering zone causes a gradual increase of the temperature therein but this increase is so slight in the upper part of the zone, in which calcining is completed, that agglomeration cannot take place. The material is raised to sintering temperature only after the tempera ture becomes uniform across the shaft cross section.

The temperature characteristics of the method of the invention differ fundamentally from those of the previously mentioned method for the calcining of lime in a co-current zone of a shaft furnace. While the heating gases at the upper end of this zone in the calcining of lime are supplied to the furnace at a temperature not exceeding 1,150C, and are at a lower temperature when the lower end of this zone is reached, the temperatures in the method according to the invention increase constantly until they have reached the required final sintering temperature (approximately l,800C for dolomite) at or near the end of the co-current zone.

The maximum temperatures which can be withstood by the conventional linings of combustion chambers of shaft furnaces of the type described hereinabove vary between approximately l,300 and l400C, depending on the kind of sintered material and its flux content, for example iron oxide, silicic acid and clay.

The shaft furnace for performing the invention as described hereinabove may be modified in many ways. Instead of a round shaft furnace as shown, it is also possible for rectangular or annular shaft furnaces to be employed. A shaft furnace having a round shaft for the calcining zone and adjoining an expanded annular shaft which forms the sintering zone as well as the cooling zone may also be used. By contrast with the described embodiments in which the burners of the gas inlet positions are disposed within combustion chambers situated outside the shaft, it is possible for the burners to be provided within cavities of bridges traversing the shaft.

Iclaim:

l. A method of calcining and sintering a material such as dolomite and magnesite in a shaft furnace, comprising the steps of a. introducing said material in the top portion of the furnace for passage downwardly there-through,

b. establishing below the zone of introduction a calcining zone with gases flowing upwardly therethrough in counter-current to the descending material,

c. establishing below said calcining zone a sintering zone with gases flowing downwardly therethrough in co-current with said descending material,

d. establishing below said sintering zone a cooling zone with gases flowing upwardly therethrough in counter-current to said descending material,

e. causing the descending gases from the sintering zone and the ascending gases from the cooling zone to be discharged from the furnace at the lower end of said sintering zone and the upper end of said cooling zone, respectively,

f. causing said discharged gases to be re-introduced into the furnace together with fuel gases at the lower end of said calcining zone to establish said counter-current flow upwardly through said calcining zone,

g. introducing a mixture of fuel gas and atmospheric air substantially devoid of furnace exhaust gases into the furnace at the top of said sintering zone to establish said co-current flow of gases downwardly through said sintering zone, and

h. adjusting the proportion of fuel gas and atmospheric air in said co-current flow of gases downwardly through said sintering zone by adding fuel or air to said mixture as needed to achieve optimum heating temperature in said sintering zone.

2. The method according to claim 1, in which said additional fuel or air is introduced into said mixture at a level below the level at which said mixture is introduced into the furnace.

3. The method according to claim 1, in which said gases discharged from the furnace at the junction of the sintering zone and the cooling zone are subjected to heat exchange with atmospheric air prior to being introduced to said calcining zone, the atmospheric air thus heated in the heat exchange process being utilized for enhancing the combustion in said calcining and sin tering zones.

4. A shaft furnace for calcining and sintering material such as dolomite and magnesite as it passes downwardly in said furnace, including means for establishing below the zone of introduction a calcining zone with gases flowing upwardly therethrough in counter-current to the descending material, means for establishing below said calcining zone a sintering zone with gases flowing downwardly therethrough in co-current with said descending material, means for establishing below said siptering zone a cooling zone with gases flowin upwardly therethrough in counter-current to said escendmg ,material, means for causing the descending gases from the sintering zone and the ascending gases from the cooling zone to be discharged from the furnace at the lower end of said sintering zone and the upper end of said cooling zone, respectively, means for causing said discharged gases to be re-introduced into the furnace together with fuel gases at the lower end of said calcining zone to establish said counter-current flow upwardly through said calcining zone, means for introducing a mixture of fuel gas and atmospheric air substantially devoid of furnace exhaust gases into the furnace at the top of said sintering zone to establish said co-current flow of gases downwardly through said sintering zone, means for adjusting the proportion of fuel gas and atmospheric air in said cocurrent flow of gases downwardly through said sintering zone by adding fuel or air to said mixture as needed to achieve optimum heating temperature in said sintering zone, upper and lower sets of combustion chambers in the furnace wall, gas discharge means for extraction of gas from said furnace at a level below said combustion chambers, conduit means connecting said discharge means with at least some of said combustion chambers for recycling extracted gas to said furnace through said combustion chambers, and means between said upper set of combustion chambers and said gas discharge means for introduction of additional fuel or air into the furnace.

5. A shaft furnace according to claim 4, in which a bridge is disposed within the furnace across the furnace walls above the lower set of combustion chambers so that the material passing down the furnace is deflected by the bridge and forms a cavity therebelow into which the lower combustion chambers communicate.

6. A shaft furnace according to claim 5, in which the means for extracting gas from the furnace are offset by relative to the lower combustion chambers.

7. A shaft furnace according to claim 5, in which the lower set of combustion chambers are disposed in the furnace wall below a zone therein where the furnace cross-section flares outwardly so that material passing down the furnace forms a cavity in and below said flared zone into which the lower combustion chambers communicate.

8. A shaft furnace according to claim 4, in which the means for supplying additional air or fuel to the furnace are disposed so as to communicate with the cavity provided below said flared zone.

9. A shaft furnace according to claim 4, in which at least one heat exchanger is disposed'in the line which recycles gas from the gas extraction means to the combustion chambers so that extracted gas may be cooled prior to introduction to the furnace, means being provided for conveying gas heated by the extracted gas in the heat exchanger to the combustion chambers. 

2. The method according to claim 1, in which said additional fuel or air is introduced into said mixture at a level below the level at which said mixture is introduced into the furnace.
 3. The method according to claim 1, in which said gases discharged from the furnace at the junction of the sintering zone and the cooling zone are subjected to heat exchange with atmospheric air prior to being introduced to said calcining zone, the atmospheric air thus heated in the heat exchange process being utilized for enhancing the combustion in said calcining and sintering zones.
 4. A shaft furnace for calcining and sintering material such as dolomite and magnesite as it passes downwardly in said furnace, including means for establishing below the zone of introduction a calcining zone with gases flowing upwardly therethrough in counter-current to the descending material, means for establishing below said calcining zone a sintering zone with gases flowing downwardly therethrough in co-current with said descending material, means for establishing below said sintering zone a cooling zone with gases flowing upwardly therethrough in counter-current to said descending material, means for causing the descending gases from the sintering zone and the ascending gases from the cooling zone to be discharged from the furnace at the lower end of said sintering zone and the upper end of said cooling zone, respectively, means for causing said discharged gases to be re-introduced into the furnace together with fuel gases at the lower end of said calcining zone to establish said counter-current flow upwardly through said calcining zone, means for introducing a mixture of fuel gas and atmospheric air substantially devoid of furnace exhaust gases into the furnace at the top of said sintering zone to establish said co-current flow of gases downwardly through said sintering zone, means for adjusting the proportion of fuel gas and atmospheric air in said co-current flow of gases downwardly through said sintering zone by adding fuel or air to said mixture as needed to achieve optimum heating temperature in said sintering zone, upper and lower sets of combustion chambers in the furnace wall, gas discharge means for extraction of gas from said furnace at a level below said combustion chambers, conduit means connecting said discharge means with at least some of said combustion chambers for recycling extracted gas to said furnace through said combustion chambers, and means between said upper set of combustion chambers and said gas discharge means for introduction of additional fuel or air into the furnace.
 5. A shaft furnace according to claim 4, in which a bridge is disposed within the furnace across the furnace walls above the lower set of combustion chambers so that the material passing down the furnace is deflected by the bridge and forms a cavity therebelow into which the lower combustion chambers communicate.
 6. A shaft furnace according to claim 5, in which the means for extracting gas from the furnace are offset by 90* relative to the lower combustion chambers.
 7. A shaft furnace according to claim 5, in which the lower set of combustion chambers are disposed in the furnace wall below a zone therein where the furnace cross-section flares outwardly so that material passing down the furnace forms a cavity in and below said flared zone into which the lower combustion chambers communicate.
 8. A shaft furnace according to claim 4, in which the means for supplying additional air or fuel to the furnace are disposed so as to communicate with the cavity provided below said flared zone.
 9. A shaft furnace according to claim 4, in which at least one heat exchanger is disposed in the line which recycles gas from the gas extraction means to the combustion chambers so that extracted gas may be cooled prior to introduction to the furnace, means being provided for conveying gas heated by the extracted gas in the heat exchanger to the combustion chambers. 